The present invention relates to a screw threaded fastener and to a structural joint achieved with said fastener when used to engage one or more relatively thin workpieces to produce clamped engagement. More specifically, the fastener member ensures that the aperture defining material of the relatively thin workpieces is drawn into wedged engagement with the driving head of the fastener, thereby limiting the degree of deformation that said aperture may undergo during clamping, and preventing disengagement of the fastener thread from the workpiece material.
Before discussing in detail the specific features of the present invention, it is believed that a brief review of the prior art and the inherent deficiencies therein will be beneficial, examples of which are shown in FIGS. 1 and 2 of the drawings. In this regard, it is only after one recognizes what is demanded in the art and what has been tried in order to fulfill this demand that the present invention can be fully appreciated and viewed in its proper perspective.
More specifically, certain problems arise during the engagement or clamping of relatively thin workpieces with conventional fasteners and these problems are well known in the art. In this regard, in order to attain clamped engagement, the workpiece must be drawn by the screw thread against the enlarged head portion of the fastener. During manufacture of conventional fasteners, a blank is provided having a shank and an enlarged head portion, the screw thread being formed by cold rolling of the shank between a pair of thread forming dies. Due to the presence of the enlarged head portion, the area of engagement of the dies with respect to the fastener shank is limited, and as such the thread immediately adjacent said enlarged head is of an incomplete or partial formation.
With relatively thick material, the presence of a partially incomplete thread turn adjacent the driving head is of little or no consequence, as the gripping function is effected by the fully formed threads on the shank displaced axially somewhat from the enlarged head. Where relatively thin sheet material is employed, as is the case in many industries, and especially the automotive industry, seeking to effect cost reduction, the gripping capacity of the thread turn adjacent the fastener head becomes of consequence. In many instances, due to the partially incomplete nature of the thread form adjacent the fastener head, gripping engagement with the workpiece is not attained, and the fastener will merely spin without drawing the workpiece into clamped relation; an occurence termed in the art "spin-out". Even if some degree of gripping engagement is effected by the thread form adjacent the fastener head, slight over torquing of the fastener will cause the sheet material or the partially formed thread to deform, thereby producing "spin-out", and losing what clamping engagement had been initially attained.
A number of solutions to the problem of spin-out have been proposed, one such solution being shown in FIG. 2 of the drawings. With respect to this prior art embodiment, the fastener head or flange is formed to a cup-shaped configuration. As such, the cup-shaped configuration overlaps the incomplete thread turn and the rim provides a surface against which the workpiece abutts during clamping. Thus, this design attempts to ensure that the partially incomplete thread turn adjacent the driving head is not relied upon to draw the workpiece into clamped engagement. While in theory this design overcomes certain of the prior art problems, in practice it has given rise to others and has not been totally satisfactory. More specifically, the depth of the cup-shaped flange has not been controlled and often was of a value equal to or greater than that of the screw pitch. Thus, as the workpiece materials become thinner, they are more easily subjected to deformation, such that over torquing or continued torquing of the fastener will result in the sheet material being drawn upwardly into the space provided by the cup-shaped flange. This action produces several undesirable results. Most importantly, deformation of the aperture defining material of the workpiece tends to enlarge the aperture, which can occur to such an extent that the gripping engagement with the thread form is destroyed, and the fastener tends to "spin-out" or strip the sheet metal material, destroying the clamped relation. Further, this movement of the workpiece material inwardly results in the partially formed or underfilled thread adjacent to the head being relied upon to effect the holding or gripping action required. As such, slight over torquing will result in loss of the clamped engagement. As still an additional factor, use of cup-shaped head also reduces the degree of frictional engagement of the fastener head with the workpiece, and a greater tendency for over torquing results.
The present invention overcomes the above-discussed prior art problems to a large extent by utilizing a cup-shaped flange or fastener head, wherein the depth of the cup-shaped configuration is closely controlled and limited. Preferably, the depth of the cup-shaped configuration is limited to approximately one-half the pitch of the thread or less, a value which has proven satisfactory in various tests and experiments. As such, the cup-shaped configuration provides a slight overlap of the thread, which decreases to a great extent the dependency upon the underfilled or partially formed thread turn adjacent the driving head for attaining clamped engagement with the workpiece. As an additional matter, the rather shallow or limited depth of the cup-shaped configuration ensures that the edge of the workpiece aperture will be drawn into contact with the base surface, and will in fact be wedged into said engagement by the screw thread. When this wedging action occurs, not only is the clamping action enhanced, but any expansion of the aperture is precluded. As a further matter, the contact of the aperture edge with the driving head serves to increase the frictional contact of the fastener with the workpiece, and prevents over torquing of the joint.
Attention is now directed to the drawings, whcih illustrate not only several embodiments of the present invention but also embodiments of the prior art. In this regard, it should be kept in mind that the illustrated embodiments are but several preferred types which have been selected for purposes of disclosure, and it is envisioned and intended that various modifications or alterations will be developed, without departing from the scope of the invention.